68 GENERAL ORNITHOLOGY. 



Homology or Analogy may be thus summed : Birds are homologically related, or natu- 

 rally allied or affiued, according to the sum of like structural characters employed for similar 

 purposes ; they are analogically related according to the sum of uulike characters employed for 

 similar purposes. A Loon and a Cormorant, for instance, are closely affined, because they are 

 both fitted in the same way for the pursuit of their prey by flying under water. A Dipper 

 (family Cinclidce) and a Loou (family Gaviidce) are analogous, in so far as both are fitted to 

 pursue their prey by flying under water ; but they stand near opposite extremes of the ornitho- 

 loffical system ; they have little affinity beyond their common birdhood ; very difi'erent struc- 

 ture being modified to attain the same end. A Crow lias vocal organs almost identical in struc- 

 ture with those of a Nightingale, and the organization of the two birds is in other respects 

 very similar ; their affinity or homology is therefore close, though the Crow is a hoarse croaker, 

 the Nightingale an impassioned musician. 



The Reason why Morphological Classification is so important as to require adoption 

 has been clearly stated by Huxley, whose words I cannot do better than quote in this connec- 

 tion. Speaking of animals, not as physiological apparatuses merely ; not as related to other 

 forms of life and to climatic conditions ; not as successive tenants of the earth ; but as fabrics, 

 each of which is built upon a certain plan, he continues : — 



" It is possible and conceivable that every animal should have been constructed upon a plan of its own, having no 

 resemblance whatever to the plan of any other animal. For any reason we can discover to the contrary, that combina- 

 tion of natural forces which we term Life might have resulted from, or been manifested by, a series of infinitely diverse 

 structures ; nor would an3i;hing in the nature of the case lead us to suspect a community of organization between ani- 

 mals so different in habit and in appearance as a porpoise and a gazelle, an eagle and a crocodile, or a butterfly and a 

 lobster. Had animals been thus independently organized, each working out its Ufe by a mechanism peculiar to itself, 

 such a classification as that now under contemplation would be obviously impossible ; a morphological or structural 

 classification plainly implying morphological or structural resemblances in the things classified. 



" As a matter of fact, however, no such mutual independence of animal forms exists in nature. On the contrary, 

 the members of the animal kingdom, from the highest to the lowest, are marvellously connected. Every animal has 

 something in common with all its fellows ; much, with many of them ; more, with a few ; and usually, so much with 

 several, that it differs but little from them. 



" Now, a morphological classification is a statement of these gradations of likeness which are observable in animal 

 structures, and its objects and uses are manifold. In the first place, it strives to throw our knowledge of the facts which 

 underlie, and are the cause of, the similarities discerned, into the fewest possible general propositions, subordinated to 

 one another, according to their greater or less degree of generality ; and in this way it answers the purpose of a memoria 

 technica, without which the mind would be incompetent to grasp and retain the multifarious details of anatomical 



"But there is a second and even more important aspect of morphological classification. Every group in that 

 classification is such in virtue of certain structural characters, which are not only common to the members of the group, 

 but distinguish it from all others ; and the statement of these constitutes the definition of the group. 



"Thus, among animals with vertebrae, the class Mammalia is definable as those which have two occipital con- 

 dyles, with a well ossified basi-occipital ; which have each ramus of the mandible composed of a single piece of bone 

 and articulated with the squamosal element of the skull ; and which possess mammae and non-nucleated red blood- 

 corpuscles. 



" But this statement of the characters of the class Mammalia is something more than an arbitrary definition. 

 It does not merely mean that naturalists agree to call such and such animals Mammalia : but it expresses, firstly, a 

 generalization based upon, and constantly verified by, very wide experience ; and, secondly, a belief arising out of that 

 generalization. The generalization is that, in nature, the structures mentioned are always found associated together ; 

 the belief is that they always have been, and always will be, found so associated. In other words, the definition of the 

 class Mammalia is a statement of a law of correlation, or coexistence, of animal structures, from which the most impor- 

 tant conclusions are deducible." (Introd. to Classif. of Animals, 8vo, London, 1869, pp. 2, 3.) 



But broad as such laws of correlation of structure are, and important as are the conclu- 

 sions deducible, we must guard against presuming upon infallibility either of the data or of the 

 deduction, as the author just quoted goes on to show. Such caution is specially required where 

 there is no obvious reason for the particular combination that may be found to exist. In the 

 case of the ostrich-like birds (Ratitce), for example, we can understand how a flat, uukeeled 

 breast-bone, a particular arrangement of shoulder-bones, and a rudimentary state of wing- 



